1. Field of the Invention
Molecular biology now offers opportunities to make a wide variety of polypeptides which previously were only chemical curiosities, since their isolation was dependent upon extraction from naturally occurring sources where they existed in only minute amounts. In many instances, the ability to prepare these polypeptides offers the availability of compounds which can be used widely in therapeutic treatments, such as vaccination and supplementing the inadequate production by a mammalian host of a particular hormone or other essential physiological regulator.
In addition, the techniques of hybrid DNA technology allow for probing the genetic inheritance of plants and mammals by cloning segments of the chromosome and sequencing and expressing specific genes. In preparing polypeptides and other products by hybrid DNA technology, there is an inherent inefficiency in that a substantial proportion of the energy must be employed for the maintenance and propagation of the cellular host. To increase the efficiency of production, it will generally be desirable to maximize the production of the desired product, in effect, diverting the cell's energies to the desired result, while still maintaining the viability of the cellular host. It is therefore desirable to develop new methods whereby greater proportions of the cellular energy is directed to production of the desired end product.
2. Description of the Prior Art
Wigler, et al. PNAS USA (1980) 77: 3567-3570 describes the transformation of mammalian cells with an amplifiable dominant-acting gene. Lee, et al. Nature (1981) 294: 228-232 describe glucocorticoid regulation of expression of dihydrofolate reductase cDNA in mouse mammary tumor virus chimeric plasmids. See also Numberg et al. PNAS USA (1978) 75: 5553-5556, and Wahl et al. J. Biol. Chem. (1979) 254: 8679-8689 for descriptions of gene amplification.